Dear Ian, LF Group,
At 21:00 15/04/2003 +0100, you wrote:
The antenna is shunt fed against ground at the 4-8ohms o/p impedance of the
BK.
Having followed this puzzling saga for some time, I have a few questions to
clarify what the situation is:
How is the loading coil connected to the ant and feeder? Is it a simple
series coil connected between coax inner and antenna down lead, or a tapped
coil with the cold end grounded, or something else? What is connected to
the antenna ground system?
What actual antenna current do you get now? Is the stated TX output power
actually being delivered to the load, or what the amplifier would give with
a matched load?
On the topic of MOSFET audio PAs, virtually any amplifier of this type can
become unstable with certain load impedances - particularly with loads that
are capacitive at high frequencies, which mis-matched coax cables can well
be. On my old audio-module based TX, I found a low-pass filter prevented
this - obviously it has to be a T - network thing with a series inductor at
the input, rather than the more usual pi network with a shunt capacitor.
Another problem was the fall-off of power at 136kHz, caused by the limited
slew rate of the input and driver stages - this can be fairly easily cured
by modifying the input stage.
Cheers, Jim Moritz
73 de M0BMU
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